The present invention relates to a cylinder lock assembly including a rotatable key plug and a key for rotating the key plug. More particularly, the present invention relates to a key having a key plug drive shoulder located and configured to minimize the torque load on the key blade as the key is turned to rotate the key plug and actuate the cylinder lock assembly.
In the design of cylinder lock assemblies, it is important that the security of the lock be maintained even when parts of the lock are subjected to deliberate excessive torque during an attack on the lock. The lock should also be designed to withstand the inadvertent application of excessive torque which, for example, could be applied by a user who attempts to turn a key forcibly to actuate the lock when the lock is jammed or otherwise inoperative.
Many conventional keys are designed to flex or even snap apart when turned too hard by a user so that excessive torque cannot be transmitted to the locking mechanism through the key and damage to the locking mechanism is prevented. In effect, the key is designed to be the weakest part of the lock to protect the structural integrity of the mechanism of the lock from damage during a deliberate or inadvertent attack using a key.
At the same time, the key should also be able to withstand normal torque applied by a user operating a lock. Many hard-to-duplicate keys of the type used in high security locks have blades which tend to bend or break during normal key usage. This problem frustrates consumers who wish to have a key that is not easily duplicated or broken and is usable in a high security lock.
A key that was configured to impart a driving force to rotate the key plug in the core without excessive flexure of the blade during normal use would be an improvement over conventional keys. This objective is especially important in situations in which security or other considerations dictate that the key blade have a cross-sectional shape that effectively reduces its torque transmission capability during normal usage. For example, in a key blade including a portion having a cross-sectional width which is thinner and torsionally weaker than similar portions of conventional keys, considerations of flexure and breakage of the key blade during normal usage may well be critical.
According to the present invention, a cylinder lock assembly includes a core, a rotatable key plug mounted in the core, and a key. The key plug is formed to include a keyway and a plurality of tumbler pin bores opening into the keyway. The key includes a bow and a blade appended to the bow. The blade includes a top edge formed to include a bitted portion for lifting the tumbler pins to a predetermined position in the tumbler pin bores to permit rotation of the key plug in the core upon rotation of a key inserted into the keyway.
The key further includes drive means for providing the primary torque to rotate the key plug relative to the core to actuate the cylinder lock assembly in response to rotation of the key about its longitudinal axis by a user seeking to unlock the cylinder lock assembly. The drive means includes a drive shoulder extending above the top edge of the blade and a bottom edge of the blade. The drive means and key plug are proportioned and configured such that the drive means drivingly engages the key plug and serves as the primary drive force for rotating the key plug. The "primary torque" or "primary drive force" is sufficient to rotate the key plug to activate the lock assembly.
In preferred embodiments, the key includes a key stop at the end of the key for engaging the key plug to position the bitted portion of the blade in alignment with tumbler pin bores opening into the keyway following insertion of the key into the keyway. The key stop is located at the tip of the blade in spaced relation to the drive shoulder. The drive shoulder is located at the throat of the key adjacent to the bow and is oriented to project away from the bottom edge of the blade to lie in an outer slot formed in the key plug.
The key stop also functions to place the drive shoulder in registry with a drive-receiving portion located in the outer slot of the key plug. The drive-receiving portion is configured to engage the drive shoulder. As the key is turned in the keyway, the drive shoulder engages the drive-receiving portion in the outer slot of the key plug to rotate the key plug in the core and actuate the lock. The drive-receiving portion of the key plug is located in a vertical slot formed in the front end of the key plug so that the drive shoulder engages a side wall of the vertical slot when the key is turned. The drive shoulder is rigid and cooperates with the bottom edge of the blade to provide the primary means for applying torque or drive force to rotate the key plug in response to turning of the key. As a result, the torque load on the rest of the blade (e.g., the bitted portion and an offset portion extending between the top and bottom edges along the length of the blade) is minimized because the rest of the blade does not carry substantial torque transmission load between the key and the key plug during turning of the key to rotate the key plug in the core. That torque load is carried primarily by the drive shoulder and the bottom edge of the blade.
One aspect of the present invention is that the torque applied by the user to rotate the key plug relative to the core and actuate the cylinder lock assembly is transmitted primarily through the drive means provided on the key instead of being transmitted only through the blade itself. In use, the stop means on the distal end of the key blade stops the key at the proper place in the keyway to position the drive shoulder in snug driving engagement with the drive-receiving surface on the key plug and the bottom edge of the blade in snug driving engagement with the key plug. Once the key is turned by a user, torque is transmitted to rotate the key plug and actuate the cylinder lock assembly by driving engagement of the drive shoulder and the bottom edge on the key against the key plug.
The drive shoulder and the bottom edge on the key are sturdy and can withstand a lot of impact during rotation of the key in the keyway by the user even if the key blade as a whole is somewhat weak and fragile. Advantageously, the drive means receives the brunt of the force applied to the key during use, thereby minimizing the magnitude of force applied to fragile or weak portions of the blade and the risk that the key blade will be bent, twisted, or broken during normal usage. Thus, the key blade of a key made in accordance with the present invention can be made to have a middle or offset portion characterized by a very thin width in cross-section because the key plug will be rotated primarily by the drive means rather than the middle or offset portion of the key blade. In many applications, it is preferable to make a key blade having a relatively thin width in cross-section in the middle of the blade because a greater variety of key blade and keyway shapes can be designed and produced.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.